1. Field of the Invention
This invention relates to communications circuits, and especially to maintenance termination units for use in telephone circuits.
2. Introduction to the Invention
In recent years, and especially in view of deregulation of many telephone systems, privately owned communication equipment has increasingly been installed in the premises of subscribers to the system, with the result that it is often necessary to determine whether any fault is located in the telephone line, or in the subscriber's premises, i.e. in the subscriber's equipment or cabling, in order to determine whose responsibility it is to repair the fault. It is highly advantageous economically if this determination can be performed remotely by sending an appropriate signal from the local exchange along the line, thereby obviating the necessity to send any telephone company personnel to the subscriber's premises.
In order to test the telephone line for any faults it is necessary firstly to install a so-called "maintenance termination unit" or MTU in the line at the subscriber's premises which can disconnect the subscriber equipment from the line (often called sectionalizing the line) and connect the a and b or tip and ring lines on receipt of the appropriate signals from the exchange. During the line testing procedure determinations will typically be made of the line to line resistance and of the first and second line to ground resistance. Also the line continuity can be determined by detecting the presence of the MTU electronically.
Various forms of MTU are described in the prior art, and may employ solid state (silicon) switches or electrical relays. The present invention is concerned with solid state switching devices in view of their greater reliability and lower cost as compared with arrangements that incorporate relays. A number of such devices are described, for example, in U.S. Pat. No. 4,710,949 to Om Ahuja. This device comprises a pair of voltage sensitive switches, one located in each of the tip and ring lines, and a distinctive termination connecting the tip and ring lines on the subscriber side of the voltage-sensitive switches. The voltage-sensitive switches may each have a threshold voltage of about 16 volts so that they are closed in normal operation by the 48 volt battery voltage but will open when this is replaced by a test voltage below about 32 volts in order to test the line-to-ground and tip-to-ring impedances. The distinctive termination may, for example, comprise a back-to-back diode and Zener diode which will exhibit an asymmetric resistance when large voltages (higher than the operating voltages) of different polarity are applied.
Although this form of MTU will perform adequately to sectionalize a fault in a line, it suffers from the problem that it requires the provision of ringing bypass capacitors in the signal path in parallel with the voltage-sensitive switches. These capacitors are necessary because the amplitude of the ringing signal (about 80 V RMS) which is superimposed on the 48 V DC battery voltage, is sufficiently large for the polarity of the resultant signal to change during the ringing signal cycles and to cause unacceptably large crossover distortion due to opening of the voltage-sensitive switches at the crossover points of the ringing signal. Because the ringing frequency is relatively low, about 20 Hz, a large capacitance is required for the ringing bypass capacitors, typically in the order of 10 .mu.F. Because the capacitors are connected in the signal line they need to have a high voltage rating in order to withstand normal electrical transients, etc, which increases their cost and physical size. In addition they can provide a low impedance path for transients.